module prime_dec(in,isprime);
  input [3:0] in; 
  output isprime;
  reg isprime;
  always @(*) begin
      case(in)
	  0,1,4,6,8,9: isprime =0;
	  2,3,5,7: isprime = 1;
	  default: isprime = 1'bx; //无关项
      endcase
  end
endmodule


